Good. Given how prone to dramatics Zarbon is, he won't admit to his idiocy immediately and Elphaba is likely to get hurt in the crossfire. So, give him the explanation that he wants to hear. "The dragon cannot be summoned because you do not have the Dragon balls." Or something to that effect.

Show him the fakes. Offer them in exchange for Elphaba. If he refuses, threaten to destroy them. Simple enough, no?